The present invention relates to a method for vaporizing and/or superheating a fuel or a fuel/water mixture for a fuel cell system. Moreover, the present invention relates to a device for carrying out such a method.
Heat exchangers which are suitable for vaporizing and/or superheating media are known from the related art. Thus, for instance, German Patent DE 44 26 692 C1 describes a heat exchanger which is composed of foils which are stacked one over another and provided with reactant channels.
In this context, a reactant flowing in a first region of the heat exchanger is heated and/or vaporized by a heat-transfer medium which flows in a second region of the heat exchanger that is in heat-conductive contact with the first region.
German Patent DE 196 39 150 C2 discloses a central heating device in which a catalytic oxidation of a fuel takes place. The thermal energy which is produced in the process can then, for example, be supplied to the heat carrier and thus be used for operating the above mentioned heat exchanger. The fuel used can be an arbitrary gaseous or liquid medium which is able to be catalytically oxidized. The examples given for the fuel in the above mentioned document are the starting material for the gas generation, in this case methanol, the product gas produced in the gas generation system or a hydrogen-containing exhaust gas of the fuel cell.
When a vaporizer of that kind is now used in a corresponding gas generation system for supply to a fuel cell system, then a very poor dynamic response ensues, in particular, in the case of step changes in load, because the vaporizer constitutes the first member in the reaction chain, while in the heating region the thermal energy is usually produced with the exhaust gas of the fuel cell and/or of the gas generation system, the vaporizer can respond to the required load change only in a lagging manner.
Located between the vaporizer and the fuel cell, which ultimately generates the requested power, are several reaction spaces of the gas generation system which each have different response times due to their structure and which, together with the fuel cell, deliver the exhaust gases required for producing the thermal energy. Due to this feedback between the vaporization region and the combustion region, therefore, the very unsatisfactory dynamic response of the overall system ensues, which turns out to be very disadvantageous for a mobile use of the fuel cell system in a motor vehicle.